The present invention relates to electronic power meters for measuring alternating current electric power and more particularly to a novel technique to provide direct current (DC) stabilization in power meters utilizing current transformers and operational amplifiers to sense current flowing in the power lines.
In the development of electronic power meters, such as watthour meters, it is conventional to utilize a current transformer in the power line to isolate the meter circuitry from the power line and to scale the current down to a value which is compatible with the electronic circuitry. The scaling factor in some power meters may be as high as 100,000 to one. Current transformers conventionally have only one or two turns in the primary circuit carrying the load current and many turns in the secondary winding whereby the current induced in the secondary winding while approximately proportional to the current flow in the primary, or in the power lines, is reduced by the ratio of the number of secondary turns divided by the number of primary turns.
However, the large power line currents flowing in the primary winding of the current transformers tend to saturate the core upon which the winding are wound. The use of larger cores or more effective core materials to prevent core saturation is undesirable because of the size and/or cost involved. It is known to utilize feedback windings on the current transformer and a feedback amplifier to induce an inverse alternating current (AC) sufficient to maintain the core magnetic flux near zero, thus keeping the core out of saturation. Since the current signals are fed through an operational amplifier, a DC offset is inherent in the amplifier characteristics and this offset may vary with factors such as time and temperature such that it cannot simply be adjusted out by variable circuit elements. Small DC effects such as amplifier offset are intolerable since they can cause the operational amplifier circuit, which may have a gain in the order of a million, to saturate which could greatly affect the accuracy of the power meter.
The use of a switching arrangement has been proposed to chop the current signal paths and provide DC stabilization of the amplifier while at the same time performing an analog multiplication function. Such an approach is shown in U.S. Pat. No. 4,761,605--Jochum, which issued on Aug. 2, 1988, and is assigned to the same assignee as the present invention. This approach requires a relatively high frequency modulated chopping signal and as a result requires a relatively wide bandwidth, high gain amplifier. Also, in an arrangement such as that shown by 4,761,605--Jochum, the relatively high frequency chopping signal necessitates better and more costly switches. In addition, since the chopping signal is modulated, it is more difficult to generate. It is desirable to provide a power meter wherein the chopping frequency can be a low frequency, to use less complex circuitry including less sophisticated operational amplifiers, and to eliminate the need for modulation and the associated pulse width modulator.